The present invention relates to a printing ink, a sheet decorated by that ink, and a synthetic resin molded article obtained by injection molding or other integral molding using that decorated sheet.
Among processes so far known to apply decorations onto the surface of a synthetic resin molded article, there is a synthetic resin molding process (hereinafter often referred to as a film insert molding process) wherein a thermoplastic resin printing material such as a thermoplastic resin film or sheet that has previously been printed by ink (hereinafter often referred to as a decorated sheet) is loaded in a mold, and a synthetic resin is then injected into the mold for integration with the decorated sheet.
This process has one feature of being capable of applying decorations even on a molded article that encounters difficulty in application of direct coloration, patterning, etc. by printing onto its surface. Another merit is that a printed layer formed on the decorated sheet can be retained between the synthetic resin molded article and a substrate film or sheet of the decorated sheet in close contact relations; much better scratch resistance is achievable as compared with printing processes for the surfaces of molded articles, and the resulting decorated article is less likely to change with years. Thus, this process is used for application of decorations onto the surfaces of various equipments inclusive of small-format electronic gadgetry such as cellular phones and PCs.
A problem with the film insert molding process is, however, that the molding synthetic resin fluidized by heating is injected at high pressure into the mold after the decorated sheet has been placed in the mold. Especially when the synthetic resin to be injected is a heat-resistant synthetic resin such as a polycarbonate resin, it is injected upon fluidization at a high temperature of 200 to 300° C. This leads to another problem that the ink printed on the decorated sheet runs out due to fluidization.
To eliminate such problems, a heat-resistant printing ink with a polycarbonate resin contained as a binder therein is used for printing of decorated sheets (see, for instance, JP2997636, JP(A)200119885, JP(A)2001294793 and JP(A)2001342398).
Even when this printing ink is used, however, an ink running problem still arises in environments in which severe injection molding conditions such as high injection rates of synthetic resins upon molding and the proximity of an injection gate to printed portions of decorated sheets are applied.
In addition, when a thin polycarbonate film or sheet is used with a decorated sheet printed by that ink, and especially when a substrate of as thin as about 125 μm is used, a printed portion is distorted or a printed surface side is largely curled. Consequently, in the case of multicolor printing, the second and subsequent colors will be printed on a distorted substrate, resulting in color misalignment and making it impossible to obtain any precisely prints. Further, in expectation of a possible curled distortion, a substrate larger than a printed area should be so provided that the printed area can be formed at only a portion of the substrate in such a way as to be not affected by the curled distortion. This offers a problem that substrate material is squandered away and workability becomes worse.
An ink that is less likely to give rise to distortion even when drying is carried out after it has been printed on a thin polycarbonate resin substrate is proposed, as set forth in JP(A)2001342398. However, there is a problem that a great deal of ink runs out during molding, although depending on where an injection gate is located at a mold.
The present invention has for its object the provision of an ink used for making a decorated sheet that is loaded in a mold upon molding for integration with a synthetic resin, which is less likely to distort or curl a thin substrate when printed thereon and dried, so that the printed ink is less likely to run out due to an injected polycarbonate resin flow and, hence, a gate can be located nearer to a printed surface of the decorated sheet.
Another object of the present invention is to provide an ink that, upon printed, has a greater adhesion force to a molded synthetic resin and enables a molded article of improved durability to be formed regardless of configuration.